mavlink: handle failure injection commands

This adds handling of MAVLink failure injection commands. An
additional parameter is added as a guard to prevent triggering any
failures by accident.

src/lib/systemlib/system_params.c

@@ -255,3 +255,16 @@ PARAM_DEFINE_INT32(SYS_HAS_BARO, 1);
  * @group System
  */
 PARAM_DEFINE_INT32(SYS_BL_UPDATE, 0);
+
+/**
+ * Enable failure injection
+ *
+ * If enabled allows MAVLink INJECT_FAILURE commands.
+ *
+ * WARNING: the failures can easily cause crashes and are to be used with caution!
+ *
+ * @boolean
+ *
+ * @group System
+ */
+PARAM_DEFINE_INT32(SYS_FAILURE_EN, 0);